Sympathomimetics

Question 1

Catecholamine Biosynthetic Pathway

Answer 1

Question 2

Alpha Adrenergic Receptors

Answer 2

Alpha-1: peripheral vasoconstriction

Alpha-2: presynaptic inhibition (inhibit NA release) in the vasomotor centre decreasing sympathetic output

Beta-1: increase function of JG cells, SA node, AV node, and the myocardium

Beta-2 : not directly innervated, activated by adrenaline in circulation causing bronchodilation, central vasodilation (heart & skeletal muscle), decreased bronchial secretion and increases glucose production

Question 3

Classification of sympathomimetics

Answer 3

Catcholamines

• Natural
  
  • Adrenaline
  • Noradrenaline
  • Dopamine
• Unnatural
  
  • Dobutamine
  • Isoprenaline

Non-Catecholamines

• Ephedrine
• Phenylephrine

Question 4

Mechanism of action

Answer 4

• Adrenaline - agonist across alpha receptors
• Noradrenaline - alpha-1 agonist (some beta-1 agonism)
• Dopamine - progressively larger doses causes stimulation of the following receptors: dopamine-1, (causing vasoconstriction), beta-1, and alpha-1
• Dobutamine - beta-1 and beta-2 agonist
• Isoprenaline - beta-1 and beta-2 agonist
• Ephedrine - agonist at alpha-1, beta-1, and beta-2 receptors (indirectly via NA release)
• Phenylephrine - alpha-1 agonist

Question 5

Indications

Answer 5

• Cardiac arrest
• Anaphylaxis (0.3 saves all!)
• Inotropic support in sepsis, hypovolemia and cardiac surgery
• Myocardial infarct (dopamine)
• Neuraxial anesthesia (ephedrine and phenylephrine)

Question 6

Contraindications

Answer 6

• Ischemic heart disease
• Occlusive vascular disease
• Diabetes mellitus
• Arrhythmia
• Hypertension
• Hyperthyroidism
• Phaeochromocytoma
• Cardiac outflow obstruction

Question 7

Drug interactions

Answer 7

• Halothane (predisposition to arrhythmia)
• Alpha and beta blockers (negate effect)
• MAOI (hypertensive crisis and arrhythmia within 2 weeks of each other)

Question 8

Side effects

Answer 8

• Increased cardiac oxygen demand (angina, MI)
• Palpitations
• Hypertension
• Arrhythmia
• Cerebrovascular accident
• Local necrosis and gangrene (central line for the big boys)
• Reflex bradycardia
• Glucose intolerance
• Pulmonary edema

Question 9

Adrenaline - Dosage

Answer 9

Cardiac arrest: 1mg (1 amp) stat

Bolus:

• Preparation: 1mg in 200ml NS (5μg/ml)
• Dose: 10-20μg

Infusion:

• Preparation: 5mg in 200ml NS (25μg/ml)
• Dose: titrate at intervals of 0.1μg/kg/min
• Tachyphylaxis at 1.5-2μg/kg/min

**1 amp = 1ml = 1mg **

Question 10

Dopamine - Dosage

Answer 10

Preparation: 200mg (1 amp) in 200ml NS (1mg/ml)

Dose: Titrated to effect; ~5μg/kg/min

Question 11

Dobutamine - Dosage

Answer 11

Preparation: 250mg (1 amp) in 200ml NS (1.25mg/ml)

Dose: titrated to effect; ~5μg/kg/min

Question 12

Ephedrine - Dosage

Answer 12

Preparation: 50mg (1 amp; 1ml) in 9ml water (5mg/ml)

Dose: 10-20mg boluses (beware tachyphylaxis)

Question 13

Phenylephrine - Dosage

Answer 13

Preparation: 10mg (1 amp) in 200ml NS (50μg/ml)

Bolus: 100-200μg

Infusion: 0.5-2μg/kg/hour